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<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><strong class="selflink"><span class="t-lines"><span>experimental::propagate_const</span></span></strong></div></div></td></tr>
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<tr class="t-nv-h1"><td colspan="5"> 成员函数</td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="propagate_const/propagate_const.html" title="cpp/experimental/propagate const/propagate const"><span class="t-lines"><span>propagate_const::propagate_const</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="propagate_const/operator%3D.html" title="cpp/experimental/propagate const/operator="><span class="t-lines"><span>propagate_const::operator=</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="propagate_const/swap.html" title="cpp/experimental/propagate const/swap"><span class="t-lines"><span>propagate_const::swap</span></span></a></div></div></td></tr>
<tr class="t-nv-h2"><td colspan="5"> 观察器</td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="propagate_const/get.html" title="cpp/experimental/propagate const/get"><span class="t-lines"><span>propagate_const::get</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="propagate_const/operator_bool.html" title="cpp/experimental/propagate const/operator bool"><span class="t-lines"><span>propagate_const::operator bool</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="propagate_const/operator%252A.html" title="cpp/experimental/propagate const/operator*"><span class="t-lines"><span>propagate_const::operator*</span><span>propagate_const::operator-&gt;</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="propagate_const/operator_element_type%252A.html" title="cpp/experimental/propagate const/operator element type*"><span class="t-lines"><span>propagate_const::operator element_type*</span><span>propagate_const::operator const element_type*</span></span></a></div></div></td></tr>
<tr class="t-nv-h1"><td colspan="5"> 非成员函数</td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="propagate_const/operator_cmp.html" title="cpp/experimental/propagate const/operator cmp"><span class="t-lines"><span>operator==</span><span>operator!=</span><span>operator&lt;</span><span>operator&gt;</span><span>operator&lt;=</span><span>operator&gt;</span><span>operator&gt;=</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="propagate_const/swap2.html" title="cpp/experimental/propagate const/swap2"><span class="t-lines"><span>swap</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="propagate_const/get_underlying.html" title="cpp/experimental/propagate const/get underlying"><span class="t-lines"><span>get_underlying</span></span></a></div></div></td></tr>
<tr class="t-nv-h1"><td colspan="5"> 辅助类</td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="propagate_const/hash.html" title="cpp/experimental/propagate const/hash"><span class="t-lines"><span>std::hash</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="propagate_const/cmp_func.html" title="cpp/experimental/propagate const/cmp func"><span class="t-lines"><span>std::equal_to</span><span>std::not_equal_to</span><span>std::less</span><span>std::greater</span><span>std::less_equal</span><span>std::greater_equal</span></span></a></div></div></td></tr>
</table></div><div></div></div></div></div><div class="t-navbar-sep"> </div></div>
<table class="t-dcl-begin"><tbody>
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<td> <div>定义于头文件 <code><a href="https://zh.cppreference.com/mwiki/index.php?title=cpp/header/experimental/propagate_const&amp;action=edit&amp;redlink=1" class="new" title="cpp/header/experimental/propagate const（页面不存在）">&lt;experimental/propagate_const&gt;</a></code>
 </div></td>
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<tr class="t-dcl t-since-libfund_ts_2">
<td> <div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T<span class="sy1">&gt;</span><br>
<span class="kw1">class</span> propagate_const<span class="sy4">;</span></span></div></td>
<td class="t-dcl-nopad">  </td>
<td> <span class="t-mark-rev t-since-libfund-ts-2 t-mark-ts">(库基础 TS v2)</span> </td>
</tr>
<tr class="t-dcl-sep"><td></td><td></td><td></td></tr>
</tbody></table>
<p><code>std::experimental::propagate_const</code> 是为指针和仿指针对象传播 const 的包装器。如名所示，在通过 <code>const</code> 访问路径访问时，它把被包装指针当做指向 <code>const</code> 的指针。
</p><p>若底层的仿指针类型类满足<a href="../named_req/MoveConstructible.html" title="cpp/named req/MoveConstructible"><i>可移动构造</i> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(MoveConstructible)</span> </a>和<a href="../named_req/MoveAssignable.html" title="cpp/named req/MoveAssignable"><i>可移动赋值</i> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(MoveAssignable)</span> </a>的要求，则该类满足对应概念，但 <code>propagate_const</code> 既非<a href="../named_req/CopyConstructible.html" title="cpp/named req/CopyConstructible"><i>可复制构造</i> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(CopyConstructible)</span> </a>亦非<a href="../named_req/CopyAssignable.html" title="cpp/named req/CopyAssignable"><i>可复制赋值</i> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(CopyAssignable)</span> </a>。
</p>
<table class="t-par-begin">


<tr class="t-par-hitem">
<td colspan="3"> 类型要求
</td></tr>
<tr class="t-par-req">
<td colspan="3"> -<div class="spacer"></div><code>T</code> 必须是下面指定的对象指针类型或仿指针类型。若 <code>T</code> 为数组类型、引用类型、指向函数指针类型、指向（可为 cv 限定的） <code>void</code> 指针类型，或若 <span class="t-c"><span class="mw-geshi cpp source-cpp">decltype<span class="br0">(</span><span class="sy2">*</span><a href="http://zh.cppreference.com/w/cpp/utility/declval"><span class="kw1120">std::<span class="me2">declval</span></span></a><span class="sy1">&lt;</span>T<span class="sy3">&amp;</span><span class="sy1">&gt;</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span></span></span> 不是左值引用类型，则程序为病式。
</td></tr></table>
<h3><span class="mw-headline" id=".E4.BB.BF.E6.8C.87.E9.92.88.E7.B1.BB.E5.9E.8B.E4.B8.8A.E7.9A.84.E8.A6.81.E6.B1.82"> 仿指针类型上的要求 </span></h3>
<p>若 <code>T</code> 为类类型，则它必须满足此节中的要求。
</p><p>给定
</p>
<ul><li> <code>t</code> ， <code>T</code> 类型的可修改<a href="../language/value_category.html" title="cpp/language/value category">左值表达式</a>
</li><li> <code>ct</code> ，绑定到 <code>T</code> 的 <code>const T&amp;</code>
</li><li> <code>element_type</code> ，对象类型
</li></ul>
<p>下列表达式必须合法并拥有其指定的效果：
</p>
<table class="wikitable">

<tr>
<th>表达式</th>
<th>返回类型</th>
<th>前提条件</th>
<th>操作语义
</th></tr>
<tr>
<td> <span class="t-c"><span class="mw-geshi cpp source-cpp">t.<span class="me1">get</span><span class="br0">(</span><span class="br0">)</span></span></span>
</td>
<td> <code>element_type*</code>
</td>
<td>
</td>
<td>
</td></tr>
<tr>
<td> <span class="t-c"><span class="mw-geshi cpp source-cpp">ct.<span class="me1">get</span><span class="br0">(</span><span class="br0">)</span></span></span>
</td>
<td> <code>element_type*</code> 或 <code>const element_type*</code>
</td>
<td>
</td>
<td> <span class="t-c"><span class="mw-geshi cpp source-cpp">t.<span class="me1">get</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1">==</span> ct.<span class="me1">get</span><span class="br0">(</span><span class="br0">)</span></span></span>
</td></tr>
<tr>
<td> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>t</span></span>
</td>
<td> <code>element_type&amp;</code>
</td>
<td> <span class="t-c"><span class="mw-geshi cpp source-cpp">t.<span class="me1">get</span><span class="br0">(</span><span class="br0">)</span> <span class="sy3">!</span><span class="sy1">=</span> nullptr</span></span>
</td>
<td> <code>*t</code> 与 <code>*(t.get())</code> 指代同一对象
</td></tr>
<tr>
<td> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>ct</span></span>
</td>
<td> <code>element_type&amp;</code> 或 <code>const element_type&amp;</code>
</td>
<td> <span class="t-c"><span class="mw-geshi cpp source-cpp">ct.<span class="me1">get</span><span class="br0">(</span><span class="br0">)</span> <span class="sy3">!</span><span class="sy1">=</span> nullptr</span></span>
</td>
<td> <code>*ct</code> 与 <code>*(ct.get())</code> 指代同一对象
</td></tr>
<tr>
<td> <span class="t-c"><span class="mw-geshi cpp source-cpp">t.<span class="me1">operator</span><span class="sy2">-</span><span class="sy1">&gt;</span><span class="br0">(</span><span class="br0">)</span></span></span>
</td>
<td> <code>element_type*</code>
</td>
<td> <span class="t-c"><span class="mw-geshi cpp source-cpp">t.<span class="me1">get</span><span class="br0">(</span><span class="br0">)</span> <span class="sy3">!</span><span class="sy1">=</span> nullptr</span></span>
</td>
<td> <span class="t-c"><span class="mw-geshi cpp source-cpp">t.<span class="me1">operator</span><span class="sy2">-</span><span class="sy1">&gt;</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1">==</span> t.<span class="me1">get</span><span class="br0">(</span><span class="br0">)</span></span></span>
</td></tr>
<tr>
<td> <span class="t-c"><span class="mw-geshi cpp source-cpp">ct.<span class="me1">operator</span><span class="sy2">-</span><span class="sy1">&gt;</span><span class="br0">(</span><span class="br0">)</span></span></span>
</td>
<td> <code>element_type*</code> 或 <code>const element_type*</code>
</td>
<td> <span class="t-c"><span class="mw-geshi cpp source-cpp">ct.<span class="me1">get</span><span class="br0">(</span><span class="br0">)</span> <span class="sy3">!</span><span class="sy1">=</span> nullptr</span></span>
</td>
<td> <span class="t-c"><span class="mw-geshi cpp source-cpp">ct.<span class="me1">operator</span><span class="sy2">-</span><span class="sy1">&gt;</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1">==</span> ct.<span class="me1">get</span><span class="br0">(</span><span class="br0">)</span></span></span>
</td></tr>
<tr>
<td> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">(</span><span class="kw4">bool</span><span class="br0">)</span>t</span></span>
</td>
<td> <code>bool</code>
</td>
<td>
</td>
<td> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">(</span><span class="kw4">bool</span><span class="br0">)</span>t</span></span> 等价于 <span class="t-c"><span class="mw-geshi cpp source-cpp">t.<span class="me1">get</span><span class="br0">(</span><span class="br0">)</span> <span class="sy3">!</span><span class="sy1">=</span> nullptr</span></span>
</td></tr>
<tr>
<td> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">(</span><span class="kw4">bool</span><span class="br0">)</span>ct</span></span>
</td>
<td> <code>bool</code>
</td>
<td>
</td>
<td> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">(</span><span class="kw4">bool</span><span class="br0">)</span>ct</span></span> 等价于 <span class="t-c"><span class="mw-geshi cpp source-cpp">ct.<span class="me1">get</span><span class="br0">(</span><span class="br0">)</span> <span class="sy3">!</span><span class="sy1">=</span> nullptr</span></span>
</td></tr></table>
<p>还有， <code>T</code> 和 <code>const T</code> 应当可按语境转换为 <code>bool</code> 。
</p><p>另外，若 <code>T</code> 可隐式转换为 <code>element_type*</code> ，则 <code>(element_type*)t</code> 应当等于 <code>t.get()</code> 。类似地，若 <code>const T</code> 可隐式转换为 <code>const element_type*</code> ，则 <code>(const element_type*)ct</code> 应当等于 <code>ct.get()</code> 。
</p>
<h3><span class="mw-headline" id=".E6.88.90.E5.91.98.E7.B1.BB.E5.9E.8B">成员类型</span></h3>
<table class="t-dsc-begin">

<tr class="t-dsc-sep">
<td colspan="2">  
</td></tr>
<tr class="t-dsc-hitem">
<td> 成员类型
</td>
<td> 定义
</td></tr>


<tr class="t-dsc">
<td>  <span class="t-c"><span class="mw-geshi cpp source-cpp">element_type</span></span>
</td>
<td>  <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="../types/remove_reference.html"><span class="kw607">std::<span class="me2">remove_reference_t</span></span></a><span class="sy1">&lt;</span>decltype<span class="br0">(</span><span class="sy2">*</span><a href="http://zh.cppreference.com/w/cpp/utility/declval"><span class="kw1120">std::<span class="me2">declval</span></span></a><span class="sy1">&lt;</span>T<span class="sy3">&amp;</span><span class="sy1">&gt;</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="sy1">&gt;</span></span></span> ， <code>T</code> 所指向对象的类型
</td></tr>
</table>
<h3><span class="mw-headline" id=".E6.88.90.E5.91.98.E5.87.BD.E6.95.B0">成员函数</span></h3>
<table class="t-dsc-begin">

<tr class="t-dsc">
<td>  <div class="t-dsc-member-div t-dsc-member-nobold-div"><div><a href="propagate_const/propagate_const.html" title="cpp/experimental/propagate const/propagate const"> <span class="t-lines"><span>(构造函数)</span></span></a></div></div>
</td>
<td>  构造新的 <code>propagate_const</code> <br> <span class="t-mark">(公开成员函数)</span> </td></tr>

<tr class="t-dsc">
<td>  <div class="t-dsc-member-div t-dsc-member-nobold-div"><div><span class="t-lines"><span>(析构函数)</span></span></div><div><span class="t-lines"><span><span class="t-mark">(隐式声明)</span></span></span></div></div>
</td>
<td>  销毁 <code>propagate_const</code> 并销毁所含指针 <br> <span class="t-mark">(公开成员函数)</span> </td></tr>

<tr class="t-dsc">
<td>  <div class="t-dsc-member-div"><div><a href="propagate_const/operator%3D.html" title="cpp/experimental/propagate const/operator="> <span class="t-lines"><span>operator=</span></span></a></div></div>
</td>
<td>  赋值 <code>propagate_const</code> 对象 <br> <span class="t-mark">(公开成员函数)</span> </td></tr>

<tr class="t-dsc">
<td>  <div class="t-dsc-member-div"><div><a href="propagate_const/swap.html" title="cpp/experimental/propagate const/swap"> <span class="t-lines"><span>swap</span></span></a></div></div>
</td>
<td>  交换被包装指针 <br> <span class="t-mark">(公开成员函数)</span> </td></tr>


<tr>
<td colspan="2"> <h5> <span class="mw-headline" id=".E8.A7.82.E5.AF.9F.E5.99.A8"> 观察器 </span></h5>
</td></tr>

<tr class="t-dsc">
<td>  <div class="t-dsc-member-div"><div><a href="propagate_const/get.html" title="cpp/experimental/propagate const/get"> <span class="t-lines"><span>get</span></span></a></div></div>
</td>
<td>  返回指向被包装指针所指向对象的指针 <br> <span class="t-mark">(公开成员函数)</span> </td></tr>

<tr class="t-dsc">
<td>  <div class="t-dsc-member-div"><div><a href="propagate_const/operator_bool.html" title="cpp/experimental/propagate const/operator bool"> <span class="t-lines"><span>operator bool</span></span></a></div></div>
</td>
<td>  检查被包装指针是否为空 <br> <span class="t-mark">(公开成员函数)</span> </td></tr>

<tr class="t-dsc">
<td>  <div class="t-dsc-member-div"><div><a href="propagate_const/operator%252A.html" title="cpp/experimental/propagate const/operator*"> <span class="t-lines"><span>operator*</span><span>operator-&gt;</span></span></a></div></div>
</td>
<td>  解引用被包装指针 <br> <span class="t-mark">(公开成员函数)</span> </td></tr>

<tr class="t-dsc">
<td>  <div class="t-dsc-member-div"><div><a href="propagate_const/operator_element_type%252A.html" title="cpp/experimental/propagate const/operator element type*"> <span class="t-lines"><span>operator element_type*</span><span>operator const element_type*</span></span></a></div></div>
</td>
<td>  到指针的隐式转换函数 <br> <span class="t-mark">(公开成员函数)</span> </td></tr>

</table>
<h3><span class="mw-headline" id=".E9.9D.9E.E6.88.90.E5.91.98.E5.87.BD.E6.95.B0">非成员函数</span></h3>
<table class="t-dsc-begin">

<tr class="t-dsc">
<td>  <div class="t-dsc-member-div"><div><a href="propagate_const/operator_cmp.html" title="cpp/experimental/propagate const/operator cmp"> <span class="t-lines"><span>operator==</span><span>operator!=</span><span>operator&lt;</span><span>operator&lt;=</span><span>operator&gt;</span><span>operator&gt;=</span></span></a></div></div>
</td>
<td>  与另一 <code>propagate_const</code> 、另一指针，或与 <span class="t-c"><span class="mw-geshi cpp source-cpp">nullptr</span></span> 比较 <br> <span class="t-mark">(函数模板)</span> </td></tr>

<tr class="t-dsc">
<td>  <div class="t-dsc-member-div"><div><a href="propagate_const/swap2.html" title="cpp/experimental/propagate const/swap2"> <span class="t-lines"><span>std::experimental::swap<span class="t-dsc-small">(std::experimental::propagate_const)</span></span></span></a></div></div>
</td>
<td>  特化 <code>swap</code> 算法 <br> <span class="t-mark">(函数模板)</span> </td></tr>

<tr class="t-dsc">
<td>  <div class="t-dsc-member-div"><div><a href="propagate_const/get_underlying.html" title="cpp/experimental/propagate const/get underlying"> <span class="t-lines"><span>get_underlying</span></span></a></div></div>
</td>
<td>  获取到被包装仿指针对象的引用 <br> <span class="t-mark">(函数模板)</span> </td></tr>
</table>
<h3><span class="mw-headline" id=".E8.BE.85.E5.8A.A9.E7.B1.BB">辅助类</span></h3>
<table class="t-dsc-begin">

<tr class="t-dsc">
<td>  <div class="t-dsc-member-div"><div><a href="propagate_const/hash.html" title="cpp/experimental/propagate const/hash"> <span class="t-lines"><span>std::hash<span class="t-dsc-small">&lt;std::experimental::propagate_const&gt;</span></span></span></a></div></div>
</td>
<td>  <code>propagate_const</code> 的哈希支持 <br> <span class="t-mark">(类模板特化)</span> </td></tr>

<tr class="t-dsc">
<td>  <div class="t-dsc-member-div"><div><a href="propagate_const/cmp_func.html" title="cpp/experimental/propagate const/cmp func"> <span class="t-lines"><span>std::equal_to<span class="t-dsc-small">&lt;std::experimental::propagate_const&gt;</span></span><span>std::not_equal_to<span class="t-dsc-small">&lt;std::experimental::propagate_const&gt;</span></span><span>std::less<span class="t-dsc-small">&lt;std::experimental::propagate_const&gt;</span></span><span>std::greater<span class="t-dsc-small">&lt;std::experimental::propagate_const&gt;</span></span><span>std::less_equal<span class="t-dsc-small">&lt;std::experimental::propagate_const&gt;</span></span><span>std::greater_equal<span class="t-dsc-small">&lt;std::experimental::propagate_const&gt;</span></span></span></a></div></div>
</td>
<td>  标准比较函数对象对 <code>propagate_const</code> 的特化 <br> <span class="t-mark">(类模板特化)</span> </td></tr>
</table>
<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">示例</span></h3>
<div class="t-example"><div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">运行此代码</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co2">#include &lt;iostream&gt;</span>
<span class="co2">#include &lt;memory&gt;</span>
<span class="co2">#include &lt;experimental/propagate_const&gt;</span>
 
<span class="kw1">struct</span> X
<span class="br0">{</span>
    <span class="kw4">void</span> g<span class="br0">(</span><span class="br0">)</span> <span class="kw4">const</span> <span class="br0">{</span> <a href="../io/cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">"g (const)<span class="es1">\n</span>"</span><span class="sy4">;</span> <span class="br0">}</span>
    <span class="kw4">void</span> g<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <a href="../io/cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">"g (non-const)<span class="es1">\n</span>"</span><span class="sy4">;</span> <span class="br0">}</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw1">struct</span> Y
<span class="br0">{</span>
    Y<span class="br0">(</span><span class="br0">)</span> <span class="sy4">:</span> m_ptrX<span class="br0">(</span><a href="../memory/unique_ptr/make_unique.html"><span class="kw735">std::<span class="me2">make_unique</span></span></a><span class="sy1">&lt;</span>X<span class="sy1">&gt;</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span> <span class="br0">{</span> <span class="br0">}</span>
 
    <span class="kw4">void</span> f<span class="br0">(</span><span class="br0">)</span> <span class="kw4">const</span>
    <span class="br0">{</span>
        <a href="../io/cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">"f (const)<span class="es1">\n</span>"</span><span class="sy4">;</span>
        m_ptrX<span class="sy2">-</span><span class="sy1">&gt;</span>g<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
    <span class="br0">}</span>
 
    <span class="kw4">void</span> f<span class="br0">(</span><span class="br0">)</span>
    <span class="br0">{</span>
        <a href="../io/cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">"f (non-const)<span class="es1">\n</span>"</span><span class="sy4">;</span>
        m_ptrX<span class="sy2">-</span><span class="sy1">&gt;</span>g<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
    <span class="br0">}</span>
 
    std<span class="sy4">::</span><span class="me2">experimental</span><span class="sy4">::</span><span class="me2">propagate_const</span><span class="sy1">&lt;</span><a href="../memory/unique_ptr.html"><span class="kw733">std::<span class="me2">unique_ptr</span></span></a><span class="sy1">&lt;</span>X<span class="sy1">&gt;&gt;</span> m_ptrX<span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
    Y y<span class="sy4">;</span>
    y.<span class="me1">f</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
 
    <span class="kw4">const</span> Y cy<span class="sy4">;</span>
    cy.<span class="me1">f</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
<p>输出：
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="text source-text"><pre class="de1">f (non-const)
g (non-const)
f (const)
g (const)</pre></div></div> 
</div>

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